Borohydride reduction of the imine groups in a pyruvate-derived cobalt(III) complex, (OC-6-33')-[Co(Aim(2)trien)](2)[ZnCl(4)], occurs with high diastereoselectivity. The major diastereoisomeric product, (OC-6-33'-ARSSR,CSRRS)-[Co(A(2)trien)]Cl has been isolated and crystallographically characterised. The results from base-induced isomerisation of the major isomer allow us to conclude that most of the remaining material from the reduction reactions (consisting of minor diastereoisomers) differs from the major isomer only in the relative configuration of the coordinated amines. Therefore the initial borohydride attack on the imine groups must have occurred predominantly on the same face of each imine as that which produces the major isomer. The diastereoselectivity of the reaction can be rationalised by proposing hydrogen bonding interactions between the incoming hydride reagent and other donor groups in the complex.